It is possible to convert wind energy to electrical energy by using a wind turbine to drive the rotor of a generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the turbine blades. To limit the energy capture at high wind speeds, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades. Matching of the variable voltage and frequency of the generator to the nominally fixed voltage and frequency of the supply network can be achieved by using a power converter.
The power converter typically includes a generator bridge, which in normal operation operates as an active rectifier to supply power to a dc link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy.
The dc output voltage of the generator bridge is fed to the dc terminals of a network bridge, which in normal operation operates as an active inverter. The principal control for the dc output voltage is achieved by controlling the generator bridge. The network bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a PWM strategy.
The ac output voltage of the network bridge is filtered and supplied to the nominally fixed frequency supply network via a step-up transformer. Protective switchgear can be included to provide a reliable connection to the supply network and to isolate the generator system from the supply network for various operational and non-operational requirements.
The power grid or supply network will operate at a nominally fixed voltage and frequency, although the latter will almost certainly vary between upper and lower limits defined in the various standards and grid codes. Such frequency variations result from power imbalances within the overall network—if load power is in excess of generated power then the frequency will fall and vice versa. Connected equipment, including that covered by the present invention, has to be able to track such frequency changes.
The PWM strategy used in the network bridge will typically operate at a given switching frequency. The mixing between the nominally fixed frequency of the power grid or supply network and the switching frequency of the PWM strategy will cause harmonics in the ac output voltage of the network bridge. In the general case, for a PWM strategy with a switching frequency Fpwm and a nominally fixed frequency of the supply network Fnet then it can be shown that significant harmonics are created at frequencies given by:Fpwm±2N*Fnetparticularly in cases where N=1 and N=2.
If Fpwm is chosen such that:Fpwm=(2M−1)*Fnetwhere M is an integer (i.e., Fpwm is an integer odd multiple of Fnet) then significant harmonics will now appear at:[(2M−1)±2N]*FnetThis means that the significant harmonics are at integer odd multiples of Fnet.
The above equations can also be explained as follows: if the nominally fixed frequency (Fnet) of the supply network is 50 Hz and the switching frequency of the PWM strategy (Fpwm) is 2.5 kHz then significant harmonics are created at 2.3 kHz, 2.4 kHz, 2.6 kHz and 2.7 kHz (i.e., 46, 48, 52 and 54 times the nominally fixed frequency of the supply network).
It is generally accepted that integer odd harmonics are preferable to integer even harmonics such as those mentioned above. This is because the allowable emissions exported into the supply network can be higher for integer odd harmonics than for integer even harmonics. For example, in the widely applied harmonics standard IEEE 519-1992 entitled “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems” the limits on current distortion for integer even harmonics are restricted to 25% of the limits for integer odd harmonics. Non-integer harmonics (or “interharmonics”) are also to be generally avoided because they can cause unwanted disturbances in electrical appliances connected to the supply network. The non-integer harmonics can also interfere with control signals that are sometimes broadcast in the interharmonic region.
One way of making sure that the mixing between the nominally fixed frequency of the power grid or supply network and the switching frequency of the PWM strategy only produces integer odd harmonics in the ac output voltage of the network bridge is to select a particular switching frequency for the PWM strategy. If the nominally fixed frequency (Fnet) of the supply network is 50 Hz then the switching frequency of the PWM strategy (Fpwm) can be set to 2.45 kHz. Significant harmonics are then created at 2.25 kHz, 2.35 kHz, 2.55 kHz and 2.65 kHz (i.e., 45, 47, 51 and 53 times the nominally fixed frequency of the supply network).
However, this solution does not take into account the fact that the nominally fixed frequency of the supply network will vary as result of power imbalances within the overall network. To this extent, the power grid or supply network must to considered to operate at a time-varying frequency even though the upper and lower limits of the variation are typically no more than about ±0.5 Hz and the changes take place gradually over the course of a number of hours or longer. However, in extreme cases, the upper and lower limits of the variation may be as much as ±3.0 Hz.
Even if the switching frequency is allowed to vary according to the time-varying frequency of the power grid or supply network then this can cause problems because the network bridge will typically have a maximum switching frequency that cannot be exceeded for thermal reasons. Extreme variations in the time-varying frequency may therefore result in a switching frequency that is higher than the maximum switching frequency such that the network bridge experiences unacceptably high thermal losses in its semiconductor switching devices.
In some cases, the choice of filter for filtering the ac output voltage of the network bridge can also place limits on the range of allowable switching frequencies for the PWM strategy. This is particularly true for tuned inductor and capacitor (LC) filters.
An improved PWM strategy for a network bridge is therefore needed.